As CMOS technology continues to scale further into the sub-micron region, forming the necessary very small feature sizes on integrated circuits is becoming increasingly difficult. Optical photolithography is currently the most widely used method for patterning these very small features during the manufacture of integrated circuits. In optical photolithography a photosensitive material known as photoresist is formed and patterned over the surface of the wafer on which the integrated circuit is being formed. In the patterning process, the pattern is transferred to the photoresist by exposing the photoresist to light through a mask called a reticle. The pattern to be transferred to the photoresist is formed on the reticle and is either transparent or opaque depending on whether positive or negative photoresist is being used. In exposing the photoresist a photolithographic tool positions the reticle above the surface of the wafer and focuses light through the reticle and unto the surface of the wafer using a complex series of lens. This type of arrangement is known as a projection optical system. In optical projection systems the pattern of the reticle is much larger than the actual pattern that is formed on the surface of the wafer and the image is reduced by the lens that make up the projection optical system. In a optical projection system the pattern is transferred to a small portion of the wafer during each exposure and either the wafer, the reticle, or both the wafer and the reticle are moved following each exposure until the pattern is transferred to the entire wafer.
A number factors combine to determine the minimum feature size that can be formed using optical photolithographic techniques. These factors include the wavelength of the light used to transfer the pattern, the numerical aperture, the resist process, etc. However, the ultimate resolution is limited by diffraction effects caused when the light passes through the pattern on the reticle. For a given wavelength of light the diffraction effects are a very important limitation in obtaining very small feature sizes. Among the various diffraction effects the most important limitation to the feature size obtainable is the optical proximity effect. The individual patterns and shapes that make up the reticle do not pattern unto the photoresist independently but interact with neighboring shapes and patterns due to the diffraction effects. A key result of the optical proximity effect is that the exact size of the shapes and patterns formed on the photoresist depend on their proximity to other shapes and patterns.
A reticle typically has a complete range of line and space sizes from the densest possible packing allowed by the lithography technology and integrated circuit design rules to very sparse packing. In the sparse packing case the pattern features are typically designated as isolated. In general a pattern feature more than 5–10 times its own size from adjacent features can be considered isolated. In the case of densely packed lines the optical proximity effect will tend to limit the minimum feature size obtainable. To correct for the optical proximity effect a number of techniques such as mask biasing, the addition of serifs, and the use of assistant pattern features have used. Shown in FIG. 1 is a region of a reticle 10 containing isolated patterns 20 with the addition serifs 30 and assist pattern features 40. The serifs 30 are added to the corners of the patterns 20 to minimize the rounding and end pull-back that would otherwise occur. The assistant pattern features 40 are placed near the isolated patterns 20 to attempt to create a dense environment around the isolated lines. The assist pattern features 40 are small enough to be below the resolution of the particular optics used. As more complex integrated circuits become necessary however different shapes such as ovals are often required. Currently there is no known method of forming these shaped structures with the necessary small feature size. In addition the assist pattern features currently in use may introduce unnecessary distortion in the patterned images. There is therefore a need for a new structure that will correct for the optical proximity effect while allowing the formation of shaped structures without introducing unnecessary distortion. The instant invention addresses this need.